Method of measuring the ph of aqueous liquids, moist foodstuffs, and the like



Sept. 11, 1951 E. w. PARKER 2,567,445

METHOD OF' MEASURING THE pH OF AQUEOUS LIQUIDS,

MOIST FOODSTUF'FS, AND THE LIKE Filed Aug. 21, 194e /l7 Cor/pons Tggiele/nen BH2 gah ail-L W. Fark EP Patented Sept. 1l,V 1951 l METHoD oFMEAsUmNG THE pn 0F AQUE- Vous zuaums, AMoIsT FoonsTUFFs,

THE LIKE Elizabeth wrarker, Washingtonjnc. "Application'August 21, 1946, Serial N0.-692',-l113 2 Claims. (C1. ,2s- 230) A l yThis invention `Vrelates to manufacture fof pH- sensitive materials and making measurements therewith; and itgco-mprises a methodvof making plastic elements in the `form of semi-permeable lmspellicles, sheets, or the like, which contain a VpH indicator dye incorporated therein; said method involving the steps :of vmixing a vpl-I ind-icator dye with a spinning solution or amoldable mass of a transparent,Y hydrophilic plastic which issemi-permeable whenformed, andthen foriningsaid lsolution or lmass into the desired shape. The invention Yalso lincludes the Vppl-I sensitive formed plastic semipermeable `lccjdics prcduccd bythe said process. In addition the invention comprises a continuous method -ojf measuring pH wherein thedescribed, semi-permeable body of plastic iscontacted with a material Awhose pl-Ikis to bc measured. and permitted .tcrcmain contact thcrcwrthsat leastyuntil:thccclcr cf. Saidduc indicates the pH of V:said material; vall as more tully-hereinaftergset fprthand asclairned.

'In-thc .prior art various :methods-hare I -hccp proposed for measuring pH by lmeans .of color indicators. The common method involvesad-ding a few jdrops of -a .solution of `an indicator rto ari-portion ofthe liquid whose pH is to be measuredand comparing the colorfproduced with 'a set of standards. .Papers impregnated with pH indicators have also been used, these papers lbeing usually dipped into the liquid to be'measured or ya fdrop of the .liquid ,being :applied to the paper. No `feasible way .of making continuouuspI-I readings by means of indicators has heen developed, however, `which has not involved zan uneconomic waste of indicator. Thus, While :suitable electrometric methods of making continuous pH measurements '.of flowing liquids are fknown, this isinot true in the case of .color indicators.

I have discovered that color indicatorfdyes can be 'readily incorporated into semi-permeable plastic materials and that, owing to the large sizes :of the dye molecules, the dyes are dissolved out-only slowly zeven in flowing water. But owing to the rapid diffusion of hydrogen and hydroxyl-'ions through these semi-permeable 4.materials thekdyes quickly and .continuously respond Jto the changes of a flowing liquor. The terlmffsemipermeable yis used 'herein in itsfzusual sense to designate a porous structurehaving poresf'so small that they substantially prevent diiusion there'- through of large molecules while permitting v.the passage of water as well -as hydrogen -and lhydroxy1 ions. Theloss of dye .'due Lto diiusion through .the A.semi-*permeable `:material fis Vfsinall and this loss can be controlled by the.'thi'cln'iess .2 ofthe plastic ami/gonny controlling A the itspores. Of coursefzthethickerthepla ment in which thcdyeisincorporated and he 1ess vits porosity, thesljowcrits rcspcnscrtc pH change, but .in most cases or continuous fmeas: urementia short time las is Inot x.ilclcftz' Hable.. Thistirne. lagfcan ,bcireduced bypassing the licucr.. WhcsepH is bicingmc urcctrapidlyfpastithc plas.- tic. clement. .'Ihc calci., ilowpf V.this tiener hasV bu t ;l ittle effect. :pppn the rate c f Lsiii-flueiep cf the diretti-rough he plastic ci thee-ye has .-becalostbv d ifusionitis @ply necessary to replace :the .-p ic withsalfrcsli indicator clcmentuwlrich rca., time .andputlittle I t. is-- b letohalance the factcrscitmc. lagfagainstfthc .costei the indicator-elem tcpbtaln resultsuvhich are satisfactcryior .purpascs @Withithc ailpf arecordipsccl- Qrim erfand controller. ;it sj i possible to control .automatically hc. plrlA c liquor is :heme measured, a new indicatcriclemcnts. .ln .making my pHgsen'sitive plastic elements any of the usual pH gindicator 1- dyes can be ernployed which are stable undenrtheparticulartconditions required to form thejplastic into-the de- Slrcd Shape. In -thecase-ot-resencratedcellulose, for examplathe.spinningsclutions usedaretrathf er highly alkaline while thespinning bath acid, so theindicator toibe inccrponatedfmustfbc able .to .withstand.thesecopditicns InthccaSc of -cellulose deriyatives., f'suh as, vcellulose esters and thers, nitrocellulose ctc.. my indicator felcf' mentsfcan be produced :by adissolyin'g these derivgafl tives in fanaorganic solvenmsmixingfin the1dye,fand then -form-ing in conventional manner; .thence these plastics .can be :used -formaking my indicator elements ,from .pf .indicator .td-ye which :is inerlt .towards `the :organic 'solvent land which has :a stabilityxsuiicient for-ordinary measurements.

Methods .dfxincorporatingtdyes ofvarious types in spinning .solutions :are-wellfknown 4in. the art. These `prior methods Ihave-lbeen usedheretof'ore for .y dying L:plastics 'fafndof f course, .lin the selec?- tion of Athe dyes -tofb'e' used, s'tabilitylef color under .allconditions i tha-sheen a primeirequisite.

IErigli-i'ca'tory fdyes, -in -contrast,#are4 notably unstable toward acids #and bases'. v:But f the mechanical methods which vhave been employed heretoiore for Sincerpor'atingl the .stable l dyes, lin. plasticsare likewise useful 'in the case rof fthe unstable @ind-ica#` tor idyes. Methods are also .known whereby formed plastic .articles can befrdyed hy contacting them with dye bathsorrtaininglfcertain. stable dyes. These methods, however, are not generally suitable in the case of my indicator element for the reason that all color indicator dyes are at least somewhat soluble in water and, if the dye penetrates into the plastic when the latter is placed in a dye bath, it will leach therefrom just as rapidly. It is possible, however, to modify this method by dyeing a formed plastic with an indicator "dye while in a gel or permeable form and to follow this with a treatment which reduces the size of the pores, rendering the plastic semipermeable, i. e. permeable to the hydroxyl and hydrogen ions but not to the dye molecules. Y

In selecting the plastics to be used a wide range is available, since any plastic can beA employed which can be dissolved or suiiiciently plasticized to be thoroughly mixed with the indicator dye and then formed into the desired shape in a state which is insoluble in the liquor whose pH is to be measured and which is semi-permeable, with pores sufficiently small as to substantially hinder diiusion of the dye molecules. Methods are known whereby satisfactory semi-permeable products can be produced from most of the wellknown plastics. Hydrophylic colloidal plastics as a group can be employed. These include regenerated cellulose, cellulose hydrate, alkali-soluble cellulose esters, hydrophilic cellulose ethers, mixed ester-ethers of cellulose, gelatine, casein, de-acetylated chitin, hydrophylic artificial resins, alkyl cellulose ethers, hydroxy-alkyl ethers, carboxy-alkyl ethers, low-nitrated cellulose nitrate, low esteried cellulose acetate and cellulose formate, cellulose ether xanthates etc. Waterswelling polymers of acrylic acid, urea-formaldehyde resins, methyl-methacrylates, polyvinyl alcohol resins in the A stage and other hydrophylic synthetic `resins can also be used.

Cellulose hydrate indicator elements can be made, for example, from solutions of viscose or cuprammoniumcellulose in which the indicator dye has been dissolved or dispersed followed by forming in the usual setting or coagulating baths. Cellulose ester indicator elements can be made by dissolving a hydrophylic organic solvent-soluble cellulose ester in an organic solvent, mixing the indicator dye therewith and spinning the mixture either in a controlled atmosphere or in a coagulating bath by methods well known in the art. Cellulose ether indicator elements can be made from alkali-soluble, water-insoluble cellulose ethers by mixing the indicator with an alkaline solution of such an ether and then forming in a suitable aqueous setting bath. Indicator elements can be made from proteins, such as gelatine or casein, by mixing the dyes in solutions of these materials, forming into films or fibers and then hardening and insolubilizing, for example by the use of a formaldehyde solution. Suitable raw materials for the production of my elements can be produced by the de-esterification of cellulose esters, for example, by the de-nitration of a high viscosity nitrocellulose. The final products can be impregnated to reduce their permeability, if desired, either with insoluble inorganic salts precipitated in situ or with colloidal matter.

, While any of the usual indicator dyes can be used in my invention, those Awhich are particularly suitable for detecting pH ,changes in the neighborhood of the neutral point are Methyl Red. Bromthymol Blue, Para-Nitrophenol, Dinitrophenol, Phenolphthalein, Meta Nitrophenol, Chlorphenol Red, Benzo Red, Meta-cresol Purple, Benzo Yellow and Brom Cresol Green. litmus can be used'if desired.

Even

While the invention is applicable to the coritinuous measurement of flowing liquors, one of the most important applications thereof is in the making of containers for perishable foodstuffs which will change color due to pH change upon the fermentation or decomposition of the foodstuffs. It is, of course, well known that most perishables emit carbon dioxide and other acrid vapors upon fermentation and decomposition. Most foods themselves also become increasingly acid. It is therefore possible to use as a wrapping for such foods a sheet of hydrophylic material in which there has been incorporated a pH indicator having the property of changing in color when contacted with acrid vapors or foodstuffs. If it is desired to use a water-proof wrapper which is nevertheless capable of indicating av change in pH, this can be accomplished by coating one side only of the described sheet with a water-impermeable coating, such as a wax or an artificial resin or varnish. One side can be coated with a glyptal resin, for example, or with a rubber chloride coating etc. The uncoated side is then placed next to the foodstuff and the wrapper Will indicate change in pH but will prevent the diffusion of moisture. Of course a separate outer impermeable wrapping can be used for the same purpose.

In the case of milk, it is possible to use a milk bottle cap or a window therein composed of one of my indicator elements in which a dye, such as Brom Cresol Purple is incorporated. This cap or window will normally be colored a gray-blue color while the milk is fresh but, upon fermentation of the milk, a yellow color of increasing brightness, -according to the acidity, will be produced. A cellophane wrapper for frozen fish or oysters, for example, impregnated with Bromthymol Blue, will normally have a blue or green color but, upon spoilage, the color will turn to yellow. If Methyl Red is substituted for the Bromthymol Blue, the color of some products will change from yellow to red upon spoilage. Such wrappers are of 4particular importance in the wrapping of frozen foods since, unless these packages are all dated, they are liable to be left in food lockers for months at a time. Prior to the present invention there was no easy way of telling whether such packages were still fit for use without opening them. But when packages are wrapped with one of my indicator sheets, a quick glance is suflicient to detect any which have deteriorated.

My invention can be explained more specilically by reference to the accompanying drawing Which shows, more or less diagrammatically, several embodiments of my indicator elements and apparatususeful in conducting my process. In this showing:

Fig. 1 is an elevation of an indicator` strip suitable for use in the continuous measurement of pH,

Fig. 2 is a side view of the indicator strip of Fig. 1, I L

Fig. 3 is a partial side view of an indicator sheet which can be used for wrapping perishables, for example,

Fig. 4 is an elevation of a milk bottle cap provided with a pI-I indicator element, in accordance with this invention,

Fig. 5 is a section through the cap of Fig. 3,

7.y is a1. side or.` 'end' View off a; niediiied:` dicator element. i

. The. legends. on the. various gures; are believed' to.. make` them. self-explanatory. Figs. 11 and 2 show elevationaland side. views, respectively, of anindi'cator element which` can beused. for gen-- eral pH. measurements, or fory measuring. con.-A tinuously the. pH of: aL flowing solution, as shown in Fig.v 6. This is merely asmall. strip of semi permeable plastic; such as. cellophane, with an indicator dye incorporated. therein. The. die me'nsions` of thisstripmay be, `for example; about. x 1i centimeters by 0.25-1 mm. thick.A For ti. tration or other work, where itv is. desired toi'obf tain rapid-measurements, of. pI-l .it isV possible to. makeI my indicator elements. of.` lm. thickness, that is-,. 0.1 mm. or lessv in thickness. ofcourse, tlie` lessi the thickness of. my indicator elements, the. more dye has to be incorporated to` produce adiStinguiShable color. But, since. my elements. can. bev made containing: up to. about. 30..,per cent of indicator dye, this-doesnot seriously limit. the dimensions of my elements. The; most serious limitation is the mechanical strength of theelements. When too thin these elementsz tend to ballup in the liquid, so. it'. is desirable to make them with suiiicient mechanicalv strength to re.-Y sistagglomeration When. immersedV in an. agitated liquid. Thismeans that the minimum thickness for'these small indicator elements is'. about; 0.51 to 0.2 mm.

AIn Fig. 3 a partial. end. view is shown of amodified i-ndicator element, inthe formof a wrappingy sheet for frozen meat or vegetables, for example. A semi-permeable.V sheet. with'. indicator dye. incorporatedis used. @ne face of thi'ssheet iscoated with a transparent water-proong material, such asi-a vinyl or glyptal resin to make the finished sheet..

The milk bottle capof Fig. 4 may be provided with the. usual pull up tab l and an indicator-disc shown generally at 2. TheV latter,A as shown in Fig. 5, may be laminated, with an inner iilm` 2 of semi-permeable -material in. which an indicator dye has. been incorporated and an outer sheet-4 ofv transparent material which, if desired, may be water-proof. An apparatus for making continuous pH measurements is shown in Fig. 6. The liquor tobe measured is introduced through.l the pipe 5its rate. of flow being controlled by the valve 6. It passes seratim through a series of measuring tubes. 1, 8 and 9 in which are placed indicator. elements I0 of the general type shown in Figs. 1 and 2. The tubes 1a, 1b, 8a, 8b, 9aV and. 9b-are blanks which .are opened only to vent. air or 'for cleaning .purposes and are used to compensate for the color of the liquor whose pI-I. is being measured. 'Ihe tubes are. closed at their tops by means of Stoppers Il. They maybe constructed of. glass or any transparent plastic. materiali The liquor is discharged through the .pipe lf2. It will be noted that, owingk to the semi-.permeable Anature of the plastic. of the indicator ele.- ments, thel-iquor whose'pH is measured.issub stantially free from any dye contamination and therefore can be united with the. main body of liquor. ThisV is an important advantagefof my invention.

Inthe making of measurements with -the--device of Fig. 6, it isVV convenient to` make. color-comparison using a comparator-showngenerally at I5. having color standards mounted therein in windows IB, every other window 'containing a comparator may be.v :convenient- 1v1 mounted, by.

cord. and can` be pulled .downL `'.scyjthat, the.. colui; standards fall inline4 with: the-,tubes of thertesltf. ing. device. The latter isA conveniently. mounted infront of aground, glass l1. The, c,ompa,risingI is;V then conducted in the. usual manner; One oi. the blanks of the comparator is` placed opposite thef` indicator tube, 8,` with :tubes 8a and 8 1ifopposits 'windows containing. the; test s tandards,. The. comparator is.Y moved along. until.4 the.` comf, bined color of the central tubes is intermediate that on either side. It is vto be noted that in thismanner compensation. is made. for any. color ofy turbidity in: the flowing. liquor whose-pH measured. In thisy .device it is important that the' indicator elements; t the.` measuring: tubes; closely 'and have; sulcient. mechanical..strengthk to. remain upright.. .Iff desired. the tubesgcan-.ibe molded` from. plasticv to .closelyv yfit the indicator elements. Itis also possible: to. mount: the. corn-ev paratorfon` a slide just back. of the testing-tubes., the slide being sufficiently long so that readings. can be takenA on any. of the measuring tubesV tori!!r The measuring tubes 1; and Scontain. indicator elements and are useful for the following reasons. As the indicator dye gradually .diffuses from the elements the` color losesI :in intensity. The-qualityf'or shade of color isnot: affectedbut pH `readings are more. .accurate when `tl`1e.=cotor intensity of 'the unknown is. approximatelyaequal tothat of the standards. In4 the device. :indicated the color intensity of lthe. indicator ele.- men'ts can be controlled. in suchl manner'that'the. central element will have approximately 'the correct intensity. This may be' accomplished. by'. providing indicator elements` which, whenrfresh, havey a color intensity which slightly'greaterl than that of thercolor standards. The indicator` elements canA lbe changedfone lata time, the fex. hausted element being removed fromv one end of the chain andi a fresh one' introduced Vat 'the opposite end, the two remaining elements being moved along the line, If'the. indicator elements are. changed in this fashion, `at the time that the last one previously introduced corresponds .in color intensity to that of.l the. color standards.. the. central lelement willgive the most accurate readf-l ings while atone side there will be an element having too great an. intensity and at .the/otherV side. one witha lower intensity than thatof the standards. If. desired,V it isl possible quickly to check all three indicator. elements: against the. standards and thus to obtain a highly accurate reading.

vIt 'evidentv that my new method :enables an accurate pH reading to .betaken with. a minimum; of `eiiort. `andzime and withoutv .the necessity lot taking,l samples, .dropping an indicator .solution in the samples, placing the samplesl in acomparatpr etc. The .latter manipulations .always require a certain degree of skill-and are usually considered as requiring the services of a chemist. With the device of the present invention, it might require the vservices-of -a chemist to change the indicator. elements, but thiswould not be required in-l general more. .often than4 oncel .in 211i hours. Inbetween .readings could be -taken by any unskilled laborerV merely by matching. the color. l

In Fig. 7 a modified indicator eIementisshoWn. This is;Y formed tof two outer semi-permeable plastic sheets cemented together with at water proof cement andwith a., p11-I. .indicator dye.y in;

tube of distilled; water,used asf-a. blank.` The. u corporated between thesheets., Thisfelementcan.

lejfornied cnveniently by mixing theV dye with the cement,- which may be a nitrocellulose c'e. ment, for example and thenusing the cement to stick the sheets together. If desired an opaque material, such as titanium dioxide or' Whiting, can be mixed in the cement. This` forms a White background for viewing the colon i The following specific examples represent embodiments of methods which can be used in producing the indicator elements of this invention.

Example lor other liquors quickly assume a color corresponding to the pH of these liquids.

Eample 2 10 .per cent solution of cellulose acetate in .acetic acid is mixed with 0.25 per cent of MetaeCresol Purple, based on the weight of the cellullose acetate. This mixture is poured into excess of water to precipitate the cellulose acetate. The precipitate is then washed and dried.. vIhe resulting powdercan be made into a molding powder from which my indicator elements can be molded, by an injection molding process, for example, or dissolved in acetone to produce a spinning solution which is then spun or formed in conventional manner in an aqueous-bath into sheets. The sheeted product produced by this method can be punched to form indicator discs and used in milk bottle caps, as shown in Figs. 4 and 5.

Eample 3 A'spinning solution of sodium cellulose-xanthogenate is mixed with Bromthymol Blue in the proportions of about 0.01 per cent of the latter based on the weight of the spun product. This is formed into strips by passing through a spinnerette into a coagulation bath containing sulfuric acid of18" B. held at a temperature of 44 C. 'Ihe strips are after-treated in an alkaline bath, rinsed and finished.

Example 4 A' cellulose acetate spinning dope containing 25 parts of cellulose acetate dissolved in 75 parts by weight of acetone is mixed with Chlorphenol Red in the amount of 0.02 per cent by weight of the latter, based on the acetate. This mixture is spun into strips, the solvent being evaporated in a stream of warm air.

Example 5 ciflc'procedures and structures which have-been described without departing from the purviewof this invention. The examples which have been given are believed suiiicient to teach those skilled in the art howto produce my indicator elements. The latter can be made in any desired form, such as filaments, strips, sheets etc. My indicator elements are particularly useful in measuring the pH of opaque liquids, such as milk, for the reason that the liquid can be shaken or wiped from the strips after immersion therein and then the strips can be checked against standards. This cannot be done with indicator papers, for example. The indicator dye can be incorporated in the plastic -by Various methods. My strips can be made with a large range of dye concentrations and, as indicated previously, it is desirable to have the dye concentration vary inversely with the thickness of the strips since it is diiiicult to see through a thick strip having a high concentration of dye. Of course such strips can be viewed by reflected light but this method is not usually as satisfactory as the usual method' of viewing them by transmitted light. I prefer to make my strips of about the same color intensity as the usual color standards used in pH colorimetric measurements, so that these standards can be used for comparison purposes. But it is possible to make strips containing all the way from about 0.005 to 20 per cent of dye, if desired. The .dye concentration (depth of color) can be lowered by soaking the indicator elements in .distilled water or a buffer solution. For strips to be viewed by reected light it is sometimes advantageous to incorporate therein some type of White opaque pigment, such as titanium dioxide. This forms strips suitable to be viewed byv reflected light since the titanium dioxide fur'- nishes a white background for the dye.V A nyV type of color standards canA be used with my strips for comparison purposes. Thus it is possible to use the conventional liquid standards formed of buler solutions sealed in glass tubes, but it is equally possible to employ colored glass strips which are available for some pH indicators.l The wedge type of color standards can also be em ployed, It is possible, of course, to have milk bottle caps formed entirely from one of my in- .dicator plastics, rather than to have merely -a window of this'material, as shown in Figs. 4 and 5. Other modications of my invention which fall Within the scope of the following claims will be immediately evident to those skilled in this art.

vWhat I claim is:

1. The process of measuring the pH of aqueous liquids, moist foodstuffs and the like, which comprises holding in direct contact with a moist ma erial Whose pH is to be measured a thin body of transparent, semi-permeable, hydrophilic, waterinsoluble colloidal plastic having uniformly incorporated in the body thereof a pH indicator dye in a concentration within the range of from about 0.005 to 20 per cent by weight; said dye changing color due to pH change over a range of pH including the pH of the material whose pH is to be measured; said body of plastic being sufciently thin'to respond quickly to changes'of pH when contacted with said material and fsufilciently porous to be permeable to hydrogen and hydroxyl ions but having pores sufllciently small substantially to hinder diffusion of the dye molecules; and comparing the color of said plastic body with pH color standards after it has been held in contactv with vsaid material fora time suflicient to assume a color corresponding to the pH of said material.

2. The process of measuring the pH of a flowing liquid which comprises immersing in the flowing liquid a thin body of transparent, semipermeable, hydrophilic, water-insoluble colloidal plastic having uniformly incorporated in the body thereof a pH indicator dye in a concentration Within the range of from about 0.005 to 20 per cent by Weight; said dye changing color due to pH change over a range of pH including the pH of the material whose pH is to be measured; said body of plastic being suiciently thin to respond quickly to changes of pH when contacted with said liquid and suiciently porous to be permeable to hydrogen and hydroxyl ions but having pores sufficiently small substantially to hinder diiusion of the ,dye molecules; and comparing the color assumed by said plastic body with pH color standards.

ELIZABETH W. PARKER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,238,123 Freeman Aug. 28, 1917 1,538,277 Dula May 19, 1925 Number Number 10 Name Date Truog Nov. 17, 1931 Snyder Nov. 21, 1933 Kiernan Apr. 21, 1936 Johnson June 30, 1936 Haeseler Dec. 8, 1936 Kenyon Nov. 16, 1937 Rosen Apr. 26, 1938 Whitehead Aug. 30, 1938 Cole Sept. 6, 1938 Sloman May 2, 1939 Dryfus Dec. 12, 1939 Snelling Sept. 10, 1940 Wenker Jan. 21, 1941 Snelling July 22, 1941 Workman July 29, 1941 Traylor Feb. 3, 1942 Klinkenstein Mar. 9, 1943 Wallach Nov. 12, 1946 Chase Jan. 25, 1949 Clark Oct. 25, 1949 FOREIGN PATENTS Country Date Great Britain July 5, 1943 OTHER REFERENCES (page 1119). 

1. THE PROCESS OF MEASURING THE PH OF AQUEOUS LIQUIDS, MOIST FOODSTUFFS AND LIKE, WHICH COMPRISES HOLDING IN DIRECT CONTACT WITH A MOIST MAERIAL WHOSE PH IS TO BE MEASURED A THIN BODY OF TRANSPARENT, SEMI-PERMEABLE, HYDROPHILLIC, WATERINSOLUBLE COLLOIDAL PLASTIC HAVING UNIFORMLY INCORPORATED IN THE BODY THEREOF A PH INDICATOR DYE IN A CONCENTRATION WITHIN THE RANGE OF FROM ABOUT 0.0005 TO 20 PER CENT BY WEIGHT; SAID DYE CHANGING COLOR DUE TO PH OF THE MATERIAL WHOSE PH PH INCLUDING THE PH OF THE MATERIAL WHOSE PH IS TO BE MEASURED; SAID BODY OF PLASTIC BEING SUFFICIENTLY THIN TO A RESPOND QUICKLY TO CHANGES TO PH WHEN CONTACTED WITH SAID MATERIAL AND SUFFICIENTLY POROUS TO BE PERMEABLE TO HYDROGEN AND HYDROXYL IONS BUT HAVING PORES SUFFICIENTLY SMALL SUBSTANTIALLY TO HINDER DIFFUSION OF THE DYE MOLECULES; AND COMPARING THE COLOR OF SAID PLASTIC BODY WITH PH COLOR STANDARDS AFTER IT HAS BEEN HELD IN CONTACT WITH SAID MATERIAL FOR A TIME SUFFICIENT TO ASSUME A COLOR CORRESPONDING TO THE PH OF SAID MATERIAL. 